Method of making nanotube permeable base transistor

ABSTRACT

A method of making a permeable base transistor (PBT) is disclosed. According to the method, a semiconductor substrate is provided, a base layer is provided on the substrate, and a semiconductor layer is grown over the base layer. The base layer includes metallic nanotubes, which may be grown or deposited on the semiconductor substrate. The nanotube base layer separates emitter and collector layers of semiconductor material.

BACKGROUND

1. Technical Field

The invention relates to permeable base transistors.

2. Discussion of Related Art

Permeable base transistors (PBTs) offer advantages in speed and packingdensity over conventional field effect transistors (FETs) (Wemersson etal., Mat. Sci. & Ens. B 51:76-80 (1998); Nilsson et al., Solid StateElec. 42:297-305 (1998)). In typical PBT technologies, a metallic baselayer is overlaid onto a single crystal semiconductor substrate(emitter/collector) to form a Schottky barrier (e.g., U.S. Pat. No.4,378,629). A second epitaxial semiconductor layer is overgrown on thebase layer (second collector/emitter). The base layer is patterned withopenings so that current can flow from emitter to collector only when avoltage is applied to the base layer. A variety of metals, such astungsten and metal silicides (e.g., WSi₂, NiSi₂ and CoSi₂) have beenused as materials for the base layer (von Känel, Mat. Sci. Rep.8:193-269 (1992); Zaring et al., Rep. Progress Phys. 56:1397-1467(1993); Pisch et al., J. App. Phys. 80:2742-2748 (1996)). These PBTswere predicted to have high gains at very high frequencies (200 GHz),which were not achievable with conventional FET technologies. However,problems such as poisoning of PBT semiconductor structures by metalelectromigration, insufficient heat dissipation, and complexity ofepitaxial overgrowth to form embedded metal base layers have preventedmass fabrication of PBTs (Hsu et al., J. App. Phys. 69:4282-4285; Miyaoet al., J. Cryst. Growth 111:957-960 (1991)).

Therefore, a need exists for new PBTs that provide the predictedimprovements in speed, packing density, and high frequency performanceover FETs, without suffering from the drawbacks associated with themetal base layers of traditional PBTs.

SUMMARY

The invention provides a method of making a permeable base transistor(PBT) having a base layer that includes nanotubes. One aspect of theinvention provides a method of making a permeable base transistor.According to the method, a semiconductor substrate is provided, a baselayer is provided on the substrate, and a semiconductor layer is grownover the base layer. The base layer includes metallic nanotubes.

In some embodiments, the base layer is formed by growing a carbonnanotube fabric on the substrate using a catalyst. In certainembodiments, the catalyst is a gas-phase catalyst. In particularembodiments, the catalyst is a metallic gas-phase catalyst. In otherembodiments, the base layer is formed by depositing a solution orsuspension of nanotubes on the substrate. In certain embodiments, thesolution or suspension is deposited by spin-coating. In particularembodiments, the solution or suspension is deposited by dipping thesubstrate into the solution or suspension. In still other embodiments,the base layer is formed by spraying an aerosol having nanotubes onto asurface of the substrate.

In some embodiments of the method, the base layer is patterned. Incertain embodiments, an ohmic contact is provided in communication withthe substrate. In particular embodiments, an ohmic contact is providedin communication with the semiconductor layer. In particularembodiments, the nanotubes include single-walled carbon nanotubes. Incertain embodiments, the base layer includes a monolayer of carbonnanotubes.

BRIEF DESCRIPTION OF THE DRAWING

In the Drawing,

FIGS. 1A-D illustrate permeable base transistor devices according tocertain embodiments of the invention;

FIGS. 2A-B illustrate nanotube fabrics of different densities used tomake certain embodiments of the invention; and

FIGS. 3-4 illustrate acts of making transistor devices according tocertain embodiments of the invention.

DETAILED DESCRIPTION

Certain embodiments of the invention provide a permeable base transistor(PBT) having a base layer including metallic nanotubes or metallicnanotube fabric embedded in a semiconductor crystal material. Themetallic nanotube base layer separates emitter and collector layers ofthe semiconductor material. A Schottky barrier is created between thenanotube layer and each of the emitter and collector semiconductorlayers. The metallic nanotube layer includes metallic nanotube species,and may also include some semiconducting nanotubes. In specificembodiments, the metallic nanotubes and semiconducting nanotubes arecarbon nanotubes. In particular embodiments, the nanotube layer is ahorizontal monolayer of single-walled carbon nanotubes. Nanotube layerscan be thinner (e.g., about 1 nm) than traditional PBT metallic basefilms, while retaining low resistance and impedance. Nanotube baselayers exhibit effective heat dissipation, due to the high thermalconductivity of the nanotubes.

In particular embodiments, PBTs of the invention are made by providing asemiconductor substrate, providing a nanotube layer on the substrate,and growing an epitaxial semiconductor layer over the nanotube layer.Epitaxial semiconductor growth over the nanotube layer can occur becausenanotubes occupy only a small surface area of the underlyingsemiconductor substrate. Since the diameter of a nanotube is only about1.5 nm, less perturbation of the epitaxial growth layer is caused by ananotube layer than by a transition metal base layer such as cobalt,nickel, or tungsten. The porous nature of the nanotube layer allows forself-sealing epitaxial overgrowth by the semiconductor, withoutgenerating the defects associated with epitaxial growth over atraditional metallic trace. Further, ultra-small patterning ofconductive base “fingers” is not required for nanotube base layers as itis for standard metallic base layers, because the nanotube fabricresembles a spider web, inherently containing openings between nanotubesegments.

FIGS. 1A-D illustrate nanotube permeable base transistors (NPBTs)according to certain embodiments of the invention. Each NPBT includes anemitter 104E, a base, and a collector 104C. In certain embodiments, asshown in FIG. 1A, the base is a metallic base layer including a carbonnanotube film 102. The thickness of the base layer corresponds to theheight of a nanotube matte, which is typically about 1 nm or greater.The nanotube base layer 102 is embedded within a crystalline matrix104C,E of semiconducting material. The semiconductor matrix 104C,E isdivided by the nanotube base layer 102 into an semiconducting emitterlayer 104E and a semiconducting collector layer 104C. A Schottky barrieris generated between the nanotube base layer 102 and each of the emitterand collector layers 104E, 104C. FIGS. 1A-D are not drawn to scale. Incertain embodiments, the thickness of each of the emitter and collectorlayers 104E, 104C is between about 0.1 microns and about 2 microns. Theemitter and collector layers 104E, 104C electrically communicate withohmic contacts 106E, 106C. FIG. 1B shows a side view of the NPBTillustrated in FIG. 1A.

In certain embodiments, as shown in FIG. 1C and its side view FIG. 1D,the base is a metallic layer including a patterned carbon nanotube film110. The patterned film 110 has nanotube “fingers” or ribbons 112separated by openings 114. The patterned film 110 may be formed usingstandard lithographic techniques to define a pattern in a nanotubelayer. Alternatively, the patterned film 110 is generated throughcontrolled growth of nanotubes to create a grid of varying nanotubedensity.

Nanotube matter and fabrics, and methods of making and patterning thesame, and of defining elements therefor, are described in co-pendingU.S. patent applications entitled Nanotube Films and Articles (Ser. No.10/128118, filed Apr. 23, 2002) and Methods of Nanotube Films andArticles (Ser. No. 10/128117, filed Apr. 23, 2002), which are assignedto the assignee of this application, and which are hereby incorporatedby reference in their entirety. A nanotube fabric is an aggregate ofnanotube segments in which nanotube segments contact other nanotubesegments to define a plurality of conductive pathways.

FIGS. 2A-B illustrate nanotube fabrics having different porosities. Theheight of the nanotubes as measured by atomic force microscopy (AFM) isabout 1-2 nm, which is typical for monolayered single-walled nanotubefilms. FIG. 2A depicts a higher density, less porous nanotube film incomparison to FIG. 2B, which depicts a lower density, more porous film.The density of the nanotube matte is adjusted using nanotube growthtechniques to provide smaller or larger openings between tubes. Forexample, the porosity of the nanotube fabric is regulated by maintaininga temperature between about 700° C. and about 1000° C. during nanotubegrowth, and allowing nanotube growth to occur for between about 1 minuteand about 1 hour. The nature and concentration of any catalyst used topromote nanotube growth, and the flow parameters of carbon precursor gasand carrier gas also affect the density of a resulting carbon nanotubematte.

Nanotube fabrics usually contain a mixture of both semiconductor andmetallic nanotube species, depending on tube chirality (Yu, et al., J.Phys. Chem. B 105:6831-6837 (2001); Erkoc et al., Int. J. Modern Phys.C, 12:865-870 (2001)). These semiconductor and metallic nanotube speciesare referred to herein as “semiconducting nanotubes” and “metallicnanotubes,” respectively. In a PBT device having a nanotube base layer,the metallic nanotubes form a Schottky contact, while the semiconductingnanotubes remain embedded as part of the epitaxial structure withoutcontributing to the device characteristics or disturbing thefunctionality of the device. The porosity of the nanotube fabric istuned to ensure that the entire nanotube layer acts as a metal trace.The nanotube layer preferably is dense enough that the carrier depletionzone generated around each metallic nanotube covers the surroundingsemiconducting nanotubes. This way the Schottky barrier created by themetallic tubes interfacing with the semiconductor matrix extends aroundthe PBT base layer.

Multiply-connected and redundant conductive paths defined by nanotubesegments in the nanotube fabric ensure reliable electrical connection ofall regions of the base layer. Redundancy in the nanotube fabric alsoreduces the detrimental effect of surface damage associated with thedeposition of epitaxial layers, which can cause delays in basetransition time (Hatzikonstantinidou et al., Phys. Scripta 54:226-229(1994)). The inherently open nature of the nanotube fabric leaves amplelattice sites exposed in the underlying semiconductor to facilitateepitaxial overgrowth in PBT fabrication.

FIG. 3 illustrates a method of making NPBT devices according to certainembodiments of the invention. The structures in FIG. 3 are shown invertical cross-section. A first intermediate structure 300 is created orprovided. In the illustrated embodiment, the structure 300 includes asemiconducting substrate having an ohmic contact 106C. In theillustrated embodiment, the semiconducting substrate provides acollector layer 104C formed of semiconductor material. However, inalternative embodiments the semiconducting substrate forms an emitterlayer. One of skill in the art will understand that the particularnature of the semiconducting substrate, e.g., n-type or p-typesemiconductor material, can be varied and is chosen based upon thestructure desired.

A nanotube layer 102′ is formed on an upper surface 302 of the substrate104C to produce a second intermediate structure 304. Usually, the mattednanotube layer 102′ is a non-woven fabric of single-walled nanotubes.However, certain alternative embodiments employ multi-walled nanotubes.In some embodiments, the nanotube film 102′ is grown using a catalyst,which is applied to the upper surface 302 of the semiconductor substrate104C. Sometimes, a catalyst metal containing iron (Fe), molybdenum (Mo),cobalt (Co), tungsten (W), or other metals, is applied to the surface302 by spin-coating or other application techniques. Alternatively, orin addition, the catalyst is provided in gaseous form in a chemicalvapor deposition (CVD) process. For example, carbon nanotubes are grownusing a gas phase metallic species such as ferrocene or other gas phasemetallic species containing iron, molybdenum, tungsten, cobalt, or othertransition metals. Parameters including, but not limited to, catalystcomposition and concentration, temperature, pressure, surfacepreparation, and growth time are adjusted to control growth of thenanotube matte 102′. For example, control of such parameters allows foreven distribution of nanotubes over the surface 302 to form a layer 102′that is primarily a monolayer of nanotubes adhered to one another viavan der Waals' forces. Growth of one nanotube on top of another occursinfrequently due to the growth tendencies of the material. In someembodiments, the catalyst is patterned to promote nanotube growth invarying densities.

Alternatively to growing the nanotube film 102′, a film 102′ ofpre-grown nanotubes is deposited on the surface 302 of the semiconductorsubstrate 104C. In some embodiments, nanotubes are dissolved orsuspended in a liquid and spin-coated over the surface 302 to generatethe nanotube film 102′. The film 102′ is one or more nanotubes thick,depending on the spin profile and other process parameters. Appropriateliquids for use in solutions or suspensions for spin-coating ofnanotubes include, but are not limited to, dimethylformamide, n-methylpyrollidinone, n-methyl formamide, orthodichlorobenzene,paradichlorobenzene, 1,2, dichloroethane, alcohols, and water withappropriate surfactants, such as, for example, sodium dodecylsulfate orTRITON X-100. The nanotube concentration and deposition parameters, suchas surface functionalization, spin-coating speed, temperature, pH, andtime, are adjusted to control deposition of monolayers or multilayers ofnanotubes as desired. In other embodiments, the nanotube film 102′ isdeposited by dipping the surface 302 of the semiconductor structure 300into a solution or suspension of nanotubes. In still other embodiments,the nanotube film 102′ is formed by spraying pre-formed nanotubes in theform of an aerosol onto the surface 302 of the semiconductor structure300.

The NPBT structure 306 is created by growing an epitaxial semiconductorlayer over the nanotube film 102′. In the illustrated embodiment, thisepitaxial semiconductor layer serves as an emitter layer 104E, but inalternative embodiments this epitaxial layer serves as a collectorlayer. Since the nanotube layer 102′ is porous, the epitaxial layer 104Egrows not only over the nanotube base layer 102′, but also betweennanotubes of the layer 102′, thereby generating a fully embeddednanotube film 102 that creates only slight perturbations in theepitaxial nature of the upper semiconductor layer 104E with respect tothe semiconductor substrate 104C. In the illustrated embodiment, theepitaxial layer 104E is interfaced to a second ohmic contact 106E. Oneof skill in the art will understand that ohmic contacts to the PBT areprovided as necessary to afford desired connections within and betweendevices.

In another embodiment, as shown in FIG. 4, an intermediate structure 304is generated as described above. The structures in FIG. 4 are shown invertical cross-section. A photoresist is applied to the nanotube layer102′ and patterned to define ribbons or fingers in the matted layer ofnanotubes 102′. The photoresist is removed to form a third intermediatestructure 400 having a patterned nanotube film 110′ made up of ribbons112′ of non-woven nanotube fabric lying on planar surface 302. Anepitaxial semiconductor layer is grown over the patterned nanotube film110′. In NPBT 402, this epitaxial semiconductor layer is shown as anemitter layer 104E, but in alternative embodiments this epitaxial layerserves as a collector layer. Since the nanotube layer 110′ is porous andextremely thin (e.g., about 1 nm), the epitaxial layer 104E grows notonly over the nanotube base layer 110′ but also between nanotubes,generating a fully embedded patterned nanotube film 110, which createsonly slight perturbations in the epitaxial nature of the uppersemiconductor layer 104E with respect to the semiconductor substrate104C. In the illustrated embodiment, the layer 104E is interfaced to asecond ohmic contact 106E. One of skill in the art will understand thatohmic contacts to the PBT are provided as necessary to afford desiredconnections within and between devices.

FIGS. 1, 3, and 4 illustrate a transistor layout according to certainembodiments of the invention, having ohmic contacts 106E, 106C above theemitter layer 104E and below the collector layer 104C. However, one ofskill in the art will appreciate that various configurations fortransistors are known, e.g., well-based, trench-based, and verticalstacking, all of which are applicable to PBTs according to variousembodiments of the invention.

It will be further appreciated that the scope of the present inventionis not limited to the above-described embodiments but rather is definedby the appended claims, and that these claims will encompassmodifications of and improvements to what has been described.

What is claimed is:
 1. A method of making a permeable base transistorcomprising: (a) providing a semiconductor substrate; (b) providing abase layer on the substrate, wherein the base layer includes metallicnanotubes and wherein the base layer is a base of the permeable basetransitor; and (c) growing a semiconductor layer over the base layer,wherein one of the semiconductor substrate and semiconductor layer is anemitter of the permeable base transistor and the other of thesemiconductor substrate and semiconductor layer is a collector of thepermeable base transistor.
 2. The method of claim 1, wherein the baselayer is formed by growing a carbon nanotube fabric on the substrateusing a catalyst.
 3. The method of claim 2, wherein the catalyst is agas-phase catalyst.
 4. The method of claim 3, wherein the catalyst is ametallic gas-phase catalyst.
 5. The method of claim 1, wherein the baselayer is formed by depositing a solution or suspension of nanotubes onthe substrate.
 6. The method of claim 5, wherein the solution orsuspension is deposited by spin-coating.
 7. The method of claim 5,wherein the solution or suspension is deposited by dipping the substrateinto the solution or suspension.
 8. The method of claim 1, wherein thebase layer is formed by spraying an aerosol having nanotubes onto asurface of the substrate.
 9. The method of claim 1, further comprisingpatterning the base layer.
 10. The method of claim 1, further comprisingproviding an ohmic contact in communication with the substrate.
 11. Themethod of claim 1, further comprising providing an ohmic contact incommunication with the semiconductor layer.
 12. The method of claim 1,wherein the nanotubes include single-walled carbon nanotubes.
 13. Themethod of claim 1, wherein the base layer includes a monolayer of carbonnanotubes.